Market App

Since ownCloud X (10.0.0) every ownCloud instance gets shipped with the
market app. This app makes it easy to manage your applications out of
the box. To connect your market app with the ownCloud Marketplace:

Get you API key under My Account

Inside the market app go to Settings

Paste your API key and click on Save

You are now able to maintain any app in
downloads/installations/updates from your ownCloud installation directly.

ownCloud Instances in Protected Environments (DMZ)

To use the market app your ownCloud instance must have an internet
connection. If your instance is running in a protected environment (DMZ
or similar) you cannot use the market app. You need to upload the apps
manually in this case. Every app can be downloaded manually from the
marketplace.